1. Field of the Invention
This invention relates generally to devices for regulating fluid transmission. More particularly, the invention relates to pressure compensating devices that also control fluid flow.
2. Description of Related Art
One of the requirements associated with many fluid transmission tasks is the need to control fluid flow. A problem often encountered when controlling fluid flow, particularly at a constant rate, is the presence of pressure variations that can adversely affect the ability to control fluid flow at a constant rate. Fluid flow rates are proportional to both flow resistance and the differential pressure across the resistance. One way of solving the pressure problem is to maintain the differential pressure across a known resistance at a fixed value. By accomplishing this, flow rate remains constant.
Numerous control mechanisms have been devised to address the pressure variation problem. All such mechanisms achieve pressure control by utilizing two restrictions, the first one variable, e.g., a reducing valve, and the second one fixed, e.g., a reference restriction. During operation of such mechanisms, the reducing valve is modulated to maintain a constant pressure differential across the reference restriction. A sensor, e.g., a piston, diaphragm or electronic sensor, monitors the pressure drop across the reference restriction and produces a signal relative to the differential pressure. The signal, (physical displacement, voltage, current, etc.), produced is used to modulate the reducing valve. Modulation of the reducing valve in response to the pressure differential across the reference restriction maintains a constant differential pressure.
The reference restriction can be a fixed orifice, variable orifice or nozzle. Desired flow rate is achieved by sizing the reference restriction accordingly.
One of the main problems with conventional pressure compensating flow control devices ("PCFC") is that they are not designed to handle low flow rates and pressures. Most PCFCs on the market today are designed to handle flow rates on the order of 0.5 to 5 gallons per minute and pressure in the 60 to 3,000 pounds per square inch (psi) range.
A further problem is sizing. Most PCFCs do not have assimilated components so their applications are limited. A yet further problem is the use of active controls to sense the pressure differentials and to electronically control the pressure-reducing valve.
Accordingly, it is an object of the present invention to eliminate the need for active controls for a PCFC. A further object is to combine the functions of multiple components into fewer components to reduce overall size for miniature applications. A yet further object is to provide a PCFC that is capable of handling low flow rates (0.0 to 0.017 gal/hr.) and low pressure rates (10-60 psi).